Gemini 31/3000

1981 — 1990
Designer
Tony Smith
Builder
Performance Cruising
Associations
?
# Built
153
Hull
Catamaran
Keel
Twin Centerboard
Rudder
?
Construction
FG

Dimensions

Length Overall
30 6 / 9.3 m
Waterline Length
27 5 / 8.4 m
Beam
14 0 / 4.3 m
Draft
1 6 / 0.5 m 4 5 / 1.4 m
Displacement
7,000 lb / 3,175 kg
Ballast
?

Rig and Sails

Type
Sloop
Reported Sail Area
425′² / 39.5 m²
Total Sail Area
?
Mainsail
Sail Area
?
P
?
E
?
Air Draft
?
Foresail
Sail Area
?
I
?
J
?
Forestay Length
?

Auxilary Power

Make
?
Model
?
HP
?
Fuel Type
Outboard
Fuel Capacity
40 gal / 151 l

Accomodations

Water Capacity
16 gal / 61 l
Holding Tank Capacity
?
Headroom
?
Cabins
?

Calculations

Hull Speed
9.1 kn
Classic: 7.03 kn

Hull Speed

The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.

Formula

Classic hull speed formula:

Hull Speed = 1.34 x √LWL

A more accurate formula devised by Dave Gerr in The Propeller Handbook replaces the Speed/Length ratio constant of 1.34 with a calculation based on the Displacement/Length ratio.

Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio.311
Hull Speed = Max Speed/Length ratio x √LWL

9.11 knots
Classic formula: 7.03 knots
Sail Area/Displacement
18.6
16-20: good performance

Sail Area / Displacement Ratio

A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.

Formula

SA/D = SA ÷ (D ÷ 64)2/3

  • SA: Sail area in square feet, derived by adding the mainsail area to 100% of the foretriangle area (the lateral area above the deck between the mast and the forestay).
  • D: Displacement in pounds.
18.58
<16: under powered
16-20: good performance
>20: high performance
Ballast/Displacement
?

Ballast / Displacement Ratio

A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.

Formula

Ballast / Displacement * 100

?
<40: less stiff, less powerful
>40: stiffer, more powerful
Displacement/Length
150.4
100-200: light

Displacement / Length Ratio

A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.

Formula

D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

  • D: Displacement of the boat in pounds.
  • LWL: Waterline length in feet
150.36
<100: ultralight
100-200: light
200-300: moderate
300-400: heavy
>400: very heavy
Comfort Ratio
11.2
<20: lightweight racing boat

Comfort Ratio

This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.

Formula

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)

  • D: Displacement of the boat in pounds
  • LWL: Waterline length in feet
  • LOA: Length overall in feet
  • Beam: Width of boat at the widest point in feet
11.24
<20: lightweight racing boat
20-30: coastal cruiser
30-40: moderate bluewater cruising boat
40-50: heavy bluewater boat
>50: extremely heavy bluewater boat
Capsize Screening
2.9
>2.0: better suited for coastal cruising

Capsize Screening Formula

This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.

Formula

CSV = Beam ÷ ³√(D / 64)

  • Beam: Width of boat at the widest point in feet
  • D: Displacement of the boat in pounds
2.93
<2: better suited for ocean passages
>2: better suited for coastal cruising

Notes

The GEMINI 31 was the first of Gemini series of cruising catamarans that became the best selling boat of its type being built in the United States. Loosely Based on the earlier ARISTOCAT 30, designed by Musters and Shaw, the 31 was superseded by the very similar Gemini 3000, which remained in production until 1990, when it was in its turn replaced by the slightly longer Gemini 3200.

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